1. Field of the Invention
The present invention relates apparatus and methods for drilling and completing a wellbore. Particularly, the present invention relates to apparatus and methods for forming a wellbore, lining a wellbore, and circulating fluids in the wellbore. The present invention also relates to apparatus and methods for cementing a wellbore.
2. Description of the Related Art
In the drilling of oil and gas wells, a wellbore is formed using a drill bit that is urged downwardly at a lower end of a drill string. After drilling a predetermined depth, the drill string and bit are removed, and the wellbore is lined with a string of casing. An annular area is thus defined between the outside of the casing and the earth formation. This annular area is filled with cement to permanently set the casing in the wellbore and to facilitate the isolation of production zones and fluids at different depths within the wellbore.
It is common to employ more than one string of casing in a wellbore. In this respect, a first string of casing is set in the wellbore when the well is drilled to a first designated depth. The well is then drilled to a second designated depth and thereafter lined with a string of casing with a smaller diameter than the first string of casing. This process is repeated until the desired well depth is obtained, each additional string of casing resulting in a smaller diameter than the one above it. The reduction in the diameter reduces the cross-sectional area in which circulating fluid may travel. Also, the smaller casing at the bottom of the hole may limit the hydrocarbon production rate. Thus, oil companies are trying to maximize the diameter of casing at the desired depth in order to maximize hydrocarbon production. To this end, the clearance between subsequent casing strings having been trending smaller because larger subsequent casings are used to maximize production. When drilling with these small-clearance casings it is difficult, if not impossible, to circulate drilled cuttings in the small annulus formed between the set casing inner diameter and the subsequent casing outer diameter.
Typically, fluid is circulated throughout the wellbore during the drilling operation to cool a rotating bit and remove wellbore cuttings. The fluid is generally pumped from the surface of the wellbore through the drill string to the rotating bit. Thereafter, the fluid is circulated through an annulus formed between the drill string and the string of casing and subsequently returned to the surface to be disposed of or reused. As the fluid travels up the wellbore, the cross-sectional area of the fluid path increases as each larger diameter string of casing is encountered. For example, the fluid initially travels up an annulus formed between the drill string and the newly formed wellbore at a high annular velocity due to smaller annular clearance. However, as the fluid travels the portion of the wellbore that was previously lined with casing, the enlarged cross-sectional area defined by the larger diameter casing results in a larger annular clearance between the drill string and the cased wellbore, thereby reducing the annular velocity of the fluid. This reduction in annular velocity decreases the overall carrying capacity of the fluid, resulting in the drill cuttings dropping out of the fluid flow and settling somewhere in the wellbore. This settling of the drill cuttings and debris can cause a number of difficulties to subsequent downhole operations. For example, it is well known that the setting of tools, such as liner hangers, against a casing wall is hampered by the presence of debris on the wall.
To prevent the settling of the drill cuttings and debris, the flow rate of the circulating fluid may be increased to increase the annular velocity in the larger annular areas. However, the higher annular velocity also increases the equivalent circulating density (“ECD”) and increases the potential of wellbore erosion. ECD is a measure of the hydrostatic head and the friction head created by the circulating fluid. The length of wellbore that can be formed before it is lined with casing sometimes depends on the ECD. The pressure created by ECD is sometimes useful while drilling because it can exceed the pore pressure of formations intersected by the wellbore and prevents hydrocarbons from entering the wellbore. However, too high an ECD can be a problem when it exceeds the fracture pressure of the formation, thereby forcing the wellbore fluid into the formations and hampering the flow of hydrocarbons into the wellbore after the well is completed.
Drilling with casing is a method of forming a borehole with a drill bit attached to the same string of tubulars that will line the borehole. In other words, rather than run a drill bit on smaller diameter drill string, the bit is run at the end of larger diameter tubing or casing that will remain in the wellbore and be cemented therein. The advantages of drilling with casing are obvious. Because the same string of tubulars transports the bit and lines the borehole, no separate trip out of or into the wellbore is necessary between the forming of the borehole and the lining of the borehole. Drilling with casing is especially useful in certain situations where an operator wants to drill and line a borehole as quickly as possible to minimize the time the borehole remains unlined and subject to collapse or the effects of pressure anomalies. For example, when forming a sub-sea borehole, the initial length of borehole extending from the sea floor is much more subject to cave in or collapse as the subsequent sections of borehole. Sections of a borehole that intersect areas of high pressure can lead to damage of the borehole between the time the borehole is formed and when it is lined. An area of exceptionally low pressure will drain expensive drilling fluid from the wellbore between the time it is intersected and when the borehole is lined. In each of these instances, the problems can be eliminated or their effects reduced by drilling with casing.
The challenges and problems associated with drilling with casing are as obvious as the advantages. For example, each string of casing must fit within any preexisting casing already in the wellbore. Because the string of casing transporting the drill bit is left to line the borehole, there may be no opportunity to retrieve the bit in the conventional manner. Drill bits made of drillable material, two-piece drill bits, pilot bit and underreamer, and bits integrally formed at the end of casing string have been used to overcome the problems. For example, a two-piece bit has an outer portion with a diameter exceeding the diameter of the casing string. When the borehole is formed, the outer portion is disconnected from an inner portion that can be retrieved to the surface of the well. Typically, a mud motor is used near the end of the liner string to rotate the bit as the connection between the pieces of casing are not designed to withstand the tortuous forces associated with rotary drilling. Mud motors are sometimes operated to turn the bit (and underreamer) at adequate rotation rates to make hole, without having to turn the casing string at high rates, thereby minimizing casing connection fatigue accumulation. In this manner, the casing string can be rotated at a moderate speed at the surface as it is inserted and the bit rotates at a much faster speed due to the fluid-powered mud motor.
Another challenge for a drilling with casing operation is controlling ECD. Drilling with casing requires circulating fluid through the small annular clearance between the casing and the newly formed wellbore. The small annular clearance causes the circulating fluid to travel through the annular area at a high annular velocity. The higher annular velocity increases the ECD and may lead to a higher potential for wellbore erosion in comparison to a conventional drilling operation. Additionally, in small-clearance liner drilling, a smaller annulus is also formed between the set casing inner diameter and the drilling liner outer diameter, which further increases ECD and may prevent large drilled cuttings from being circulated from the well.
A need, therefore, exists for apparatus and methods for circulating fluid during a drilling operation. There is also a need for apparatus and methods for forming a wellbore and lining the wellbore in a single trip. There is a further need for an apparatus and methods for circulating fluid to facilitate the forming and lining of a wellbore in a single trip. They is yet a further need to cement the lined wellbore.